In a top-swinging construction machine such as a hydraulic excavator, an upper structure is swingably attached to a lower vehicle body (base carrier) provided with a carrier. A working equipment including a boom, an arm, a bucket or the like is pivotally mounted to the upper structure. The base carrier is driven by a travel hydraulic motor while the upper structure is swung by a hydraulic swing motor. The boom, arm, bucket and the like are pivoted respectively by a boom cylinder, an arm cylinder, a bucket cylinder and the like.
Pressure oil discharged from a variable displacement hydraulic pump driven by an engine is supplied to or discharged from hydraulic actuators of the hydraulic motors and the cylinders via control valves provided corresponding to the actuators. The pump displacement of the variable displacement hydraulic pump is controlled according to a load pressure and a pump discharge pressure of the hydraulic actuator and the position of the control valve.
For instance, the pump displacement of the hydraulic pump is controlled in accordance with a load-sensing differential pressure between the load pressure of the hydraulic actuator and the discharge pressure of the hydraulic pump. In addition, the pump displacement of the hydraulic pump is controlled so that a pump absorption torque (pump displacement of the hydraulic pump×pump discharge pressure of the hydraulic pump) becomes a predetermined value or less.
Specifically, when the hydraulic actuator requires a large amount of pump discharge flow in accordance with the load-sensing differential pressure, the pump displacement of the hydraulic pump is controllably enlarged. On the other hand, when the hydraulic actuator does not require a large amount of pump discharge flow or the control valve is returned to a neutral position (i.e. a position at which the pressure oil is not supplied to the hydraulic motor and the cylinder), the pump displacement of the variable displacement hydraulic pump is controllably reduced.
The pump displacement is controlled so that the pump is capable of discharging a flow rate required by the hydraulic actuator. By thus controlling the pump displacement of the variable displacement hydraulic pump in accordance with load-sensing differential pressure, when the pressure oil is not necessary to be supplied to the hydraulic actuators such as those of the hydraulic motor and the cylinder, the pump displacement of the variable displacement hydraulic pump can be set at the minimum. Accordingly, the consumption power of the engine for driving the variable displacement hydraulic pump can be reduced.
A target pump displacement for controlling the pump displacement of the hydraulic pump can be set, for instance, in accordance with a relationship between a target pump absorption torque of the hydraulic pump and the pump discharge pressure of the hydraulic pump or in accordance with an operation amount of a control lever for operating a hydraulic swing motor for driving the upper structure.
Generally, a relational expression of D=T/P is established between a pump displacement D, pump absorption torque T and pump discharge pressure P of a hydraulic pump. Though a constant is required between a right-hand member and a left-hand member of the relational expression, the constant is omitted in the above relational expression. According to the relational expression, the target pump displacement corresponding to the current pump discharge pressure P can be determined in accordance with the target pump absorption torque T. Incidentally, the target pump absorption torque is generally set in accordance with the engine speed at each time period.
Alternatively, the target pump displacement corresponding to the operation amount of the control lever for operating the hydraulic swing motor may be determined through experiments or the like, thereby setting the target pump displacement corresponding to detected operation amount of the control lever. In accordance with the detected operation amount of the control lever, a swash plate angle of the hydraulic pump can be controlled so that the pump displacement of the hydraulic pump is set at the target pump displacement.
The target pump absorption torque of the hydraulic pump is thus controlled, so that, the pump displacement is reduced when the pump discharge pressure is high and, the pump displacement is increased when the pump discharge pressure is low. The target pump absorption torque of the hydraulic pump is set in accordance with the output condition of the engine (full output and partial output). Since the target pump absorption torque is thus controlled, the overload of the engine for driving the variable displacement hydraulic pump and consequent engine failure are prevented.
When, for instance, a hydraulic swing motor for driving the upper structure of a hydraulic excavator is exemplified, an operation on a pilot valve for swing movement switches a position of a control valve for the hydraulic swing motor (this control valve for hydraulic swing motor will be referred to as a swing control valve hereinafter) from a neutral position to feed the pressure oil discharged from the hydraulic pump toward the hydraulic swing motor. Then, the upper structure of the hydraulic excavator is swung by the drive of the hydraulic swing motor.
When the swing control valve of the hydraulic swing motor is switched, the pump displacement of the hydraulic pump is controlled to be a pump displacement corresponding to the load-sensing differential pressure (a differential pressure between a pump discharge pressure and a load pressure of the hydraulic swing motor) applied on a load sensing valve for controlling the pump displacement of the hydraulic pump. In other words, when the swing control valve is switched, the hydraulic pump is immediately (normally within approximately 0.2 to 0.3 second) controlled so as to increase the pump displacement.
Incidentally, the same function also works not only in the above load-sensing hydraulic circuit but also in an open-center hydraulic circuit.
However, since an inertial force for keeping the upper structure at halt is large, it takes some time before the upper structure is accelerated from the halted state to a steady swing velocity (a state at which all of the pump discharge amount commanded by the swing control valve flows toward the hydraulic swing motor). Normally two to three seconds are required for a startup time required for accelerating from the halted state to the steady swing velocity.
Accordingly, during the time until the upper structure is accelerated to the steady swing velocity, a part of the pressure oil discharged from the hydraulic pump is not used for driving the hydraulic motor but is discharged from a swing relief valve to be wasted as an extra flow during the acceleration of the upper structure is accelerating. The wasteful discharge of the pressure oil discharged from the hydraulic pump results in deterioration of fuel consumption of the engine, a temperature increase in the hydraulic oil, an increase in relief noise and the like.
In order to control the relief flow rate, a hydrostatic drive device (see Patent Document 1), a hydraulic circuit of a construction machine (see Patent Document 2), a hydraulic control device of a hydraulic working equipment (see Patent Document 3) and the like are proposed. The solution disclosed in Patent Document 1 applies a swing acceleration pressure on a side of a springbox of a swing control valve (referred to as a parallel narrowing part in Patent Document 1) opposite to a spool-drive side. The spool of the swing control valve is returned to a position at which the swing acceleration pressure and the spring force are balanced to reduce the relief flow rate.
The solution disclosed in Patent Document 2 employs a regulator for controlling a pump displacement of a variable displacement hydraulic pump of an open-center hydraulic circuit. The regulator is controlled by higher one of: remnant discharge pressure of a discharge pressure from a hydraulic pump after being used by an actuator; and a pilot pressure outputted by a proportional solenoid valve controlled by a controller. The controller outputs a command signal for controlling the proportional solenoid valve in accordance with a detection value of the pump discharge pressure discharged by the variable displacement hydraulic pump.
When the controller detects an operation on a swing control valve (referred to as a switch control valve in Patent Document 2), the controller outputs to the proportional solenoid valve a pilot pressure for reducing the pump displacement of the variable displacement hydraulic pump in accordance with the detected pump discharge pressure.
Patent Document 3 discloses a hydraulic pressure controller for a hydraulic working equipment that is adapted to cutting off a discharge flow rate of a variable displacement hydraulic pump for supplying pressure oil for driving an actuator, in which a relief valve of a swing motor is provided by a variable swing relief valve. When a working pressure exceeds a cutoff set pressure, an absorption torque of the variable displacement hydraulic pump is decreased. When the absorption torque of the variable displacement hydraulic pump is decreased, the relief pressure of the variable swing relief valve is increased by a predetermined pressure.
[Patent Document 1] JP-A-1982-116966
[Patent Document 2] JP-A-2003-294003
[Patent Document 3] JP-A-2001-50202